Organic EL display panels include luminescent elements that utilize the electroluminescent properties of certain organic compounds (organic EL elements).
An organic EL display panel is manufactured by arranging onto a substrate a matrix of sub-pixels (organic EL elements) of three different colors: red (R), green (G), and blue (B). Each set of R, G, and B organic EL elements constitutes one pixel. The respective organic EL elements are manufactured by arranging, in order, pixel electrodes (e.g., anodes), organic EL layers, and counter electrodes (e.g., cathodes) onto a substrate. In some cases, functional layers such as electron injection layers, electron transport layers, hole transport layers, and/or hole injection layers are also formed.
The organic EL elements are typically of three types: organic EL element R which emits red light; organic EL element G which emits green light; and organic EL element B which emits blue light. In some cases, all of the organic EL layers may have white light-emitting luminescent layers while providing color filters that change white light to desired colored light. In other cases, a red light-, green light-, or blue light-emitting organic EL layer are provided for each of the organic EL elements.
Functional layers including organic EL layers, hole injection layers, and hole transport layers are formed for instance by applying coating solutions of functional layer materials onto a substrate and drying the coating solutions. More specifically, banks made of resin or the like are formed over the surface of the substrate to define regions for each of R, G and B, where such functional layers are to be provided. Subsequently, coating solutions are applied in the corresponding regions defined by the banks and dried to form functional layers.
When forming functional layers by coating method in this way, it is possible that the drying rate of coating solution differs between the center and surrounding edge coating region of the panel. The drying rate of a coating solution influences the shape of a functional layer formed. That is, variations in drying rates result in variations in shape profiles and thicknesses of functional layers to be formed. The variations in functional layer thickness among pixels lead to brightness variations across the display.
In an effort to overcome this problem, technologies have been proposed in which the coating region (a region where functional layers are to be formed) at the edge of the panel (edge coating region) is made larger than the coating region at the center of the panel (center coating region) (see Patent Literature 1 listed below). In Patent Literature 1, the edge coating region is made larger than the center coating region, allowing the larger coating region to hold more coating solution than the smaller one. In this manner, the difference in drying rate of coating solution between the center and edge coating regions of the panel is corrected.
Other proposed technologies involve setting the amount of solvent contained in the coating solution applied in the edge coating region of the panel to be larger than the amount of solvent contained in the coating solution applied on the center coating region, so that the difference in drying rate of the coating solution between the center and edge coating regions is corrected (see, e.g., Patent Literature 2).
Yet other proposed technologies involve providing a pixel electrode-free region (dummy region) that surrounds a luminescent region consisting of a matrix of pixels, so that brightness variation across the display that occurs due to the difference in drying rate between the center region and surrounding edge region of the panel can be avoided (see, e.g., Patent Literatures 3 to 6).
Providing such a dummy region around the perimeter of the luminescent region and applying a coating solution in the dummy region in this way results in the formation of functional layers of varying thickness in the dummy region. However, the dummy region can reduce variations in drying rates of coating solutions across the luminescent region positioned at the center of the panel, so that the functional layers to be formed in the luminescent region are uniform in thickness among pixels. In this manner, it is possible to reduce brightness variations across the display.
It should be also noted that functional layers to be formed exhibit different shape profiles depending on the characteristics of the solute and solvent of their coating solution as well as on the ratios of the solute and solvent in the coating solution. For example, when color filters that correspond to the respective colors of organic EL elements are to be formed by coating, coating solutions of the color filters require different solutes depending on the color of the color filter. Thus, when the wall (bank) height and/or bank taper angle are uniform across the substrate, the shape of the color filter tends to vary from color to color. A technology that aims to overcome this problem changes the bank height and/or bank taper angle depending on the color of the color filter, so that the color filters exhibit increased uniformity in thickness (see, e.g., Patent Literature 7).
Moreover, a technology for enhancing light extraction efficiency from organic EL elements employs a transparent electrode for either of the pixel electrode and counter electrode; and employs a reflective electrode for the other while disposing a transparent conductive film by sputtering or the like between the organic EL layer and reflective electrode (see, e.g., Patent Literature 8). By appropriately adjusting the optical distance between the organic EL layer and reflective electrode by means of the transparent conductive film disposed between them, the light beam reflected by the reflective electrode and then travels toward the transparent electrode and the light beam that directly travels toward the transparent electrode are combined together to increase light extraction efficiency.